CEL P0420; Steps for Diagnosis

[Donabed Kopoian]

If you have more than 3 full warm up and cool down cycles in those 2 weeks (and most likely you did), then you are set.

I replace the upstream HO2S every 60,000 miles for optimum economy and to keep the engine running best. The downstream I don't touch, until it fails.

The wheel being off balance will transmit that vibration through the brake lines and into the brake pedal if it is bad enough. Hell ... when the clutch slave cylinder was dying in my Focus, I could feel the pop in the clutch pedal every time a prong broke off.

Do what Kevin said, rotate your tires, see if you notice a difference, and also, my money is on the wheel that was "straightened" as the culprit.

 

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Thanks for the advice everyone.

I'm going to wait another week or so and then sign this one off.

Hopefully it will be accessed by others in the future who have the dreaded P0420 CEL code. They will know to start with spark plugs and advance to the other diagnostics advice in this thread.

Good luck everyone in the future with their car care needs. You'll see me on future threads for sure.

 

[Kev250R]

I'm glad it worked-out! And thanks for updating the thread, that will definitely help others who may stumble across this thread in a Google search.

Kevin

 

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All,

Okay, I'm closing this one up. In conclusion, my CEL came on. Code P0420.

I had it "diagnosed" by a mechanic who said both O2 sensors were bad and the catalytic converter (from a statistical perspective this is close to impossible).

I joined the Corolla Forum and was given sound advice on diagnosing the issue.

I changed my spark plugs and downstream O2 sensor at ~87K miles.

The CEL is still OFF.

End of thread.